Effect of aspect ratio on the nonlinear optical properties of ZnO nanorods

In this work, we synthesized ZnO nanorods with two different aspect ratios using the solution method. Morphological and crystalline structures were investigated using TEM and XRD analysis. Also investigated the effect of aspect ratio on the optical properties of ZnO nanorods. With an increasing aspect ratio of the nanorods, the absorption peak is red-shifted and near-band edge emission is suppressed. The nonlinear absorption is enhanced with the increasing aspect ratio of ZnO nanorods.


Introduction
Zinc Oxide (ZnO) is a widely used semiconductor with a wide direct bandgap, high mobility, large exciton binding energy, and high chemical and thermal stability [1,2].ZnO is used in numerous scientific fields due to its excellent electro-optical, chemical, and mechanical properties [1,3].By changing the synthesis conditions, controlled synthesis of ZnO various morphologies are possible such as nanoparticles, nanotubes, nanoneedles, nanoflower, nanorods, nanoplate, nanowire, nanopellets, etc[1,2].Among these nanostructures, ZnO nanorods have attracted great attention because of their wide applications in the light detector, LED, lasers, sensors, solar cells, and photocatalysis [2,4].In this paper, we synthesized ZnO nanorods with two different aspect ratios and examined their linear and nonlinear optical properties.

Preparation method
Ultrasound sonication-assisted simple solution method was employed in the preparation of ZnO nanorods.0.1M Zn(CH3COO)2.2H2Oand 0.5M NaOH were dissolved in selected solvents.A transparent white solution was formed and heated at 80℃ for 2 hours and kept at room temperature for 24 hours.The resulting white precipitate was centrifuged and dried to get in powder form.Figure 1 shows the schematics of the synthesis procedure.

Material Characterizations
A transmission electron microscope (TEM; JEM 2100) was used to analyze the morphology.The crystallinity of the samples was characterized by XRD (Bruker AXS D8 Advance).The absorption of prepared ZnO nanorods was measured using a JASCO V-570 UV-vis spectrometer and emission spectra was analyzed using a Cary Eclipse Varian spectrofluorometer.

Nonlinear optical studies
First, we prepared a stable colloidal ZnO nanofluid.For this synthesized nanorods in the powder form were dispersed in water at the concentration of 0.1mg/ml under ultrasonication.These colloids were used as a sample for the nonlinear studies.The nonlinear optical studies were carried out using the Zscan method, in which a Q-switched Nd-YAG laser (532nm, 7ns, 10Hz) was used as an irradiation source.Z-scan is a highly sensitive and simple technique for estimating the nonlinear absorption of the sample.The sample was fixed on the translational stage and moved along the beam focus, selffocusing/ defocusing effects caused the change in transmittance value.When the sample is irradiated with the beam, the transmittance at focus either decreases or increases depending on the property of the material.More details of Z scan techniques have been described in our previous publications[3].

Results and Discussion
TEM images of the ZnO nanorods synthesized in 1-Butanol and 1-Hexanol are shown in Figure 2. The nanorod synthesized using 1-Butanol and 1-Hexanol was named as Sample A and Sample B. The TEM image revealed the formation of nanorod-like morphology.Longer nanorods are formed in 1-Hexanol (Sample B) compared with 1-Butanol (Sample A) with an aspect ratio of 5 and 2 respectively.The aspect ratio of nanorod increases with increasing alkyl group length of the selected solvent.More details for the growth mechanism were explained in our previous publication[2].The crystal structure of nanorods was studied using XRD analysis shown in Figure 3. Two samples exhibited a hexagonal wurtzite structure of ZnO in agreement with JCPDS file number 01-089-1397.The peaks corresponding to the planes are identical and no peak shift or other crystalline phases are detected.The UV-vis absorption spectra and emission spectra are shown in Figure 4.As the aspect ratio increases absorption peak is shifted to a higher wavelength region.The absorption spectra exhibited blue shift and band edge absorption [2].Generally, the photoluminescence spectra include two bands.The first band represents the UV emission and the second band is a broad visible state emission.In Figure 4(B), only near-band edge emission is observed for both samples.The intensity of emission intensity increases with decreasing the aspect ratio.This increment is due to the presence of minimum lattice surface defects.Lattice surface defects act as a nonradiative recombination trapping center, which increases the surface recombination and decreases the near band edge emission [5].
Here q0 is the fitting parameter.As the aspect ratio of ZnO nanorods increases, the depth of the valley increases.This increment in depth indicates the enhancement in nonlinear behavior.Here larger nanorods (Sample B) possess higher nonlinear optical properties.Calculated nonlinear absorption (β) is 600 cm/GW and 760 cm/GW for Sample A and Sample B respectively.In this work, synthesized nanorods lie in a weak confinement regime, and the exciton oscillation strength increases with an increase in particle size.Thus, the enhancement in nonlinear absorption (β) is related to the quantum confinement effect as well as the exciton oscillator strength[3].

Conclusion
The present study revealed the aspect ratio-dependent linear and nonlinear optical properties of ZnO nanorods.According to the increase in aspect ratio, the UV absorption peak is shifted towards the higher wavelength region and increases the surface defects which suppressed the UV emission.In the case of nonlinear optical studies increase in aspect ratio enhances the nonlinear absorption.This study confirmed that the optical properties of ZnO nanostructures are controlled by changing their aspect ratio.
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